ISTANBUL, Turkey—Jelena Jankovic calls out Serena Williams for a potential lack of sportsmanship in her three-set loss to the American in the semifinal of the WTA Championships. Williams suffered physically in the match, at times having little power on her shots and not moving well.

“I never saw her do this when she’s leading,” Jankovic said. “Every time she has a lead, she’s running for the balls and she’s hitting those big serves. For some reason, every time she starts losing, she starts serving slower or not running for certain balls. That’s something that as a player you have to pay attention.

“It’s not the first time when we played that she’s doing this. That’s I think her way of playing or maybe when she plays against me. But when it’s important moment, huge serve comes up, so you have to be on alert every time. You never know if she’s going to come slow or really fast one. It’s not easy to focus. It’s important for me to focus on my game and not really let those kind of things affect me and not allow me to play my game, but it’s the way it is. She’s by far I think the best player. She deserves to be in this spot. But I think it’s also, when you play, winning or losing, you have to be up there and be a good sportsman.”